Optical fiber with multi section core

ABSTRACT

An optical fiber comprising a core region embedded within a cladding. The core region of the optical fiber further comprises multiple sections, each doped with rare earth ions. The sections of the core region may be doped with different rare-earth ions or with different doping concentrations. The sections of the core region may also be made from different types of glass hosts. The optical fiber may further include multiple core regions embedded within the cladding, each core region having multiple sections doped with rare earth ions.

BACKGROUND

Various implementations, and combinations thereof, are related tooptical fiber, and more particularly to optical fiber with multi coresections.

Generally speaking, an optical fiber is a fiber of glass or plasticcapable of carrying light along its length and typically comprising acore section surrounded in a cladding, as illustrated in FIG. 1. Anoptical beam is propagated through the length of fiber 100 via a core102, confined therein by a cladding 104, which has a lower refractiveindex than the core.

The core itself can either be single mode or multimode. Multimode fiberssupport multiple transverse modes, whereas single mode fibers supportonly one. Typically, multimode fibers are used for communication linksover short distances or where high power transmissions are desired. Incontrast, single mode fibers are used for long-distance communicationlinks. The relationship between the mode of fiber, the core, and theoptical beam is given by the following equation:

V=2π×NA×a/λ

where NA is the numerical aperture, a is the radius of the core, and λis the wavelength of the optical beam. When V is less than or equal to2.405, the fiber is a single mode fiber. Otherwise, it is a multimodefiber.

When doped with rare-earth ions, such as neodymium or ytterbium, opticalfibers can be used as the gain medium in fiber lasers or fiberamplifiers. Such fiber is generally referred to as a gain fiber and therare-earth ions are doped in the core and/or the cladding. Differentlaser wavelengths are generated from fibers with different doping ions.For example, approximately 1 and 1.3 microns are achieved with neodymiumdoped fibers, 1.55 and 2.7 microns from erbium doped fibers, 1 micronfrom ytterbium doped fibers, and 2 microns from thulium and/or holmiumdoped fibers.

Different types of gain fibers are designed for use in different fiberlasers, the characteristics of the gain fiber effecting the resultingfiber laser. For example, the use of a double cladding gain fiberincreases the output power of a fiber laser. Several existing patentsfocus on this relationship by attempting to affect the quality andnature of fiber lasers through the development of the gain fiber. By wayof example, U.S. Pat. No. 4,829,529, issued to Kafka provides a singlemode fiber laser pumped by a coherent high power laser diode source.Kafka attempts to address the issue that the small diameter of singlemode fibers limits the ability to couple such fibers to high poweredcoherent laser diode sources, resulting in low powered lasers, whereasmultimode fibers are not so limited, but the resulting lasers have poorbeam quality output. Specifically, Kafka discloses laser diode pumpedfiber lasers with double cladding. FIG. 2 provides an exemplaryembodiment of a fiber 200 as used in Kafka having a double claddingwhere a core 206 embedded within an inner cladding 204 and an outercladding 202. The optical fiber used has rare-earth ions doped into thecore to provide an active gain medium. A multimode pump laser is coupledto the inner cladding to increase the pump power and excite therare-earth ions in the core of the fiber. The larger cross section ofthe inner cladding, in comparison to the core, allows the multimodelaser to be coupled to a single mode fiber. As a result, the high pumppower of the inner cladding compared to the core pump produces a fiberlaser having high output.

By way of another example, U.S. Pat. No. 5,566,196, issued to Scifres,attempts to provide an optical fiber laser or amplifier medium usingmultimode fibers and having an increased output power without producingnonlinear optical effects such as Brillouin scattering. The fiber lasersand amplifiers of Scifres employ optical fibers with two or moregenerally parallel, nonconcentric doped core regions, each of which iscapable of gain or lasing when optically pumped. An exemplary opticalgain fiber according to Scifres is illustrated in FIG. 3. The fiber 300may be single clad or double clad, the single clad fiber having only theinner cladding 304 where as the double clad fiber additionally has theouter cladding 302. Multiple cores 306 may be embedded in a commoncladding region, such as inner cladding 304, or in separate claddingregions. The use of multiple cores spreads the light over a larger areaof the fiber, compared with a single mode fiber, and thereby reducing oreliminating the non linear optical effects that would otherwise occur athigh light intensities.

The cores of the gain fibers of Kafka and Scifries are formed with arelatively uniform area. The laser is generated and/or amplified by oneor more rare-earth ions doped into the core of the fiber. Other ions maybe used to transfer energy to the lasing ions. For example, ytterbiumions are sometimes doped into an erbium doped fiber. The resultinglasing ions are erbium and receive energy transferred from the ytterbiumafter absorbing the pump. However, by forming gain fibers with arelatively uniform area, only one wavelength is generated from eachfiber. In other words, the laser only occurs in one transition from theupper level to the lower level from one lasing ion.

Clearly, the development of fiber lasers and fiber amplifiers, such asthose described in either Kafka or Scifres, would benefit from opticalfibers having non uniform cores, each core section being doped withdifferent rare-earth ions and resulting in a gain fiber capable ofgenerating more than one wavelength.

SUMMARY

In one implementation, an optical fiber is presented having a coreregion embedded within a cladding. The core region further has multiplesections, each of which is doped with a rare-earth ion.

In another implementation, an optical fiber is presented having multiplecore regions embedded within a cladding. Each core region further hasmultiple sections, each of which is doped with a rare-earth ion.

In another implementation, a fiber laser is presented. The fiber laserincludes a laser optical fiber having a core region embedded within acladding. The core region further has multiple sections, each of whichis doped with a rare-earth ion.

In yet another implementation, an ultra short pulse fiber laser ispresented. The ultra short pulse fiber includes a laser optical fiberhaving a core region embedded within a cladding where the core regionfurther has multiple sections. The laser optical fiber is additionallydoped with a plurality of rare-earth ions.

In still another implementation, a broad band amplified spontaneousemission (“ASE”) source is presented. The broad band ASE source includesa first core region embedded within a cladding. The core region furtherhas multiple sections, each of which is doped with a rare earth ion.

BRIEF DESCRIPTION OF THE DRAWINGS

Implementations of the invention will become more apparent from thedetailed description set forth below when taken in conjunction with thedrawings, in which like elements bear like reference numerals.

FIG. 1 is a cross sectional view of an exemplary embodiment of anoptical fiber having a core and a single cladding;

FIG. 2 is a cross sectional view of an exemplary embodiment of anoptical fiber having a double cladding;

FIG. 3 is a cross sectional view of an exemplary embodiment of anoptical fiber having multiple cores;

FIG. 4 is a cross sectional view of an exemplary embodiment of anoptical fiber having two core sections;

FIG. 5 is a cross sectional view of an exemplary embodiments of theconfigurations of a core having multiple sections;

FIG. 6 is a cross sectional view of an exemplary embodiment of anoptical fiber perform;

FIG. 7 is a schematic depiction of an exemplary embodiment of a fiberlaser using an optical fiber having multiple cores according to thepresent discussion.

DETAILED DESCRIPTION

Implementations propose an optical fiber having multiple core sections,which, when used, cause a laser to be generated and/or amplifiedsimultaneously from more than one lasing ions and/or from more than onetransition. Throughout the following description, this invention isdescribed in preferred embodiments with reference to the figures inwhich like numbers represent the same or similar elements. Referencethroughout this specification to “one embodiment,” “an embodiment,” orsimilar language means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least on embodiment of the present invention. Thus, appearances ofthe phrases “in one embodiment,” “in an embodiment,” and similarlanguage throughout this specification may, but do not necessarily, allrefer to the same embodiment.

The described features, structures, or characteristics of the inventionmay be combined in any suitable manner in one or more embodiments. Inthe following description, numerous specific details are recited toprovide a thorough understanding of embodiments of the invention. Oneskilled in the relevant art will recognize, however, that the inventionmay be practiced without one or more of the specific details, or withother methods, components, materials, and so forth. In other instances,well-known structures, materials, or operations are not shown ordescribed in detail to avoid obscuring aspects of the invention.

Turning to the Figures, FIG. 4 depicts a cross sectional view of anexemplary embodiment of an optical fiber 400 having core sections 404and 406, surrounded by a cladding 402. A person of ordinary skill in theart will realize that, although FIG. 4 depicts an optical fiber havingtwo core sections, the following discussion is equally applicable tooptical fibers having more than two core sections.

In one embodiment, by dividing the core into sections, each core section404 and 406 can be doped with different rare-earth ions. Thus, by way ofexample, core section 404 may generate a different wavelength when usedin a fiber laser then core section 406.

In other embodiments, core sections 404 and 406 may be made fromdifferent glass host materials. By way of example, and not by way oflimitation, core section 404 can be silicate glass while core section406 is phosphate glass. In yet other embodiments, core sections 404 and406 may be doped with the same rare-earth ions but with different dopingconcentrations, thus resulting in different spectroscopic propertiesand, therefore, affecting the performance of a fiber laser.

As stated, the present discussion is applicable to optical fibers havingany number of core sections. Further, the core sections can have avariety of configurations. FIG. 5 presents a cross sectional view ofexemplary embodiments of different configurations of segmented cores. Aperson of ordinary skill in the art will realize that FIG. 5 is providedby way of illustration, and not by way of limitation, and that thepresent discussion encompasses configurations other than those depictedin FIG. 5.

In one embodiment, although the core is formed having multiple discretesections, from the point of view of a propagating optical beam, thesections form a single mode guide. In such an embodiment, each sectionof the core may be formed from glass having approximately the samerefractive index. In other words, the difference between the refractiveindices of the various sections is much smaller than the differencebetween the refractive index of the core and that of the cladding.

A person of ordinary skill in the art will realize that although thepresent discussion focuses on single mode fibers and although a singlemode fiber will be sufficient for most applications, the presentdiscussion is equally applicable to multimode fibers. In such anembodiment, a cross sectional view of an individual multi section coreof a multimode fiber will appear the same as a cross sectional view of amulti section core of a single mode fiber. However, the relationshipbetween the mode of fiber, the core, and the optical beam is no longerdesignated by the equation:

V=2π×NA×a/λ.

By way of example, an optical fiber having a multi section coreaccording to the present discussion may be fabricated by doping one coreglass with erbium ions, having a refractive index of 1.50 at 1.55microns. A second core glass may be doped with thulium ions, having arefractive index of 1.50±0.0005 at 1.55 microns. Thus, the differencebetween the two core glasses is less than 0.001. The cladding class maybe fabricated having a refractive index of 1.4935, resulting in anumerical aperture of 0.14.

Each core glass may be ground until they form semi-cylindrical rods,such that when the grounded surfaces are disposed in contact with oneanother, the two core glasses form a cylinder. The ground surface ofeach core glass may then be polished.

A cladding glass tube can be fabricated from the cladding glass suchthat the inner diameter of the tube equals the outer diameter of thecore glass, the outer diameter of the cladding being dependent upon thesize of the fiber. Both the inner and outer diameters are polished. Oncedone, the semi-cylindrical rods of erbium doped glass and thulium dopedclass are inserted into the inner diameter of the cladding glass tube toform the fiber preform. FIG. 6 is a cross sectional view of an exemplaryfiber perform 600 fabricated according to the present discussion.

The fiber preform is placed into a fiber drawing tower. During the fiberdrawing process, the two core glasses are physically bonded to form thesingle mode core area.

As stated, an optical fiber having a multi section core according to thepresent discussion can be used to generate multiple laser wavelengthssimultaneously. For example, using the fiber of FIG. 6, a 1.55 micronfiber laser can be generated in the erbium doped core section and a near2 micron fiber laser in the thulium doped core section. An exemplaryembodiment of a fiber laser 700 using such an optical fiber is depictedin FIG. 7.

The ability to generate multiple laser wavelengths in the same singlemode core is highly beneficial. By way of example, and not by way oflimitation, one wavelength can be used as the pump source and the otheras the probe wavelength in a pump and probe experiment. As anotherexample, two wavelengths can be used to generate a new laser wavelengththrough a nonlinear process such as, for example, different frequencygeneration, frequency summing, and frequency doubling.

A fiber having a multi section core can also be used to generate anultra short pulse fiber laser. The pulse width of a short pulse fiberlaser is limited by the bandwidth of the gain medium. By doping multiplerare-earth ions into a fiber having a multi section core, gain bandwidthcan be effectively extended thereby allowing an extremely short pulsefiber laser to be achieved.

A fiber according to the present discussion can additionally be used togenerate extremely broad band amplified spontaneous emission (“ASE”)source. By way of example, and not by way of limitation, one section ofa bisected core can be doped with thulium ions and other with holmiumions. As is known by those of ordinary skill in the art, thulium emitsemissions from 1.7 to 1.9 microns and holmium from 1.9 to 2.1 microns.By doping the stated sections with thulium ions and holmium ionsrespectively, an ASE source with emissions from 1.7 to 2.1 microns canbe generated.

The present invention may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. The describedimplementations are to be considered in all respects only asillustrative and not restrictive. The scope of the invention is,therefore, indicated by the appended claims rather than by the foregoingdescription. All changes which come within the meaning and range ofequivalency of the claims are to be embraced within their scope.

While the preferred embodiments of the present invention have beenillustrated in detail, it should be apparent that modifications andadaptations to those embodiments may occur to one skilled in the artwithout departing from the scope of the present invention as set forthin the following claims.

1. An optical fiber, comprising: a core region embedded within acladding, the core region having a plurality of sections, wherein eachof the plurality of sections is doped with a rare-earth ion.
 2. Theoptical fiber of claim 1, wherein at least one said section of theplurality of sections is doped with a first rare-earth ion and at leastone other said section is doped with a second rare-earth ion, the firstrare-earth ion being different from the second rare earth ion.
 3. Theoptical fiber of claim 1, wherein at least one said section of theplurality of sections is doped with a first doping concentration and atleast one other said section is doped with a second dopingconcentration, the first doping concentration being different from thesecond doping concentration.
 4. The optical fiber of claim 1, wherein atleast one said section of the plurality of sections is of a first glasshost and a at least one other section is of a second glass host, thefirst glass host being different from the second glass host.
 5. Anoptical fiber, comprising: a plurality of core regions embedded within acladding, each of the said plurality of core regions having a pluralityof sections, wherein each of the plurality of sections is doped with arare-earth ion.
 6. The optical fiber of claim 5, wherein at least onesaid section of the plurality of sections of one said core region of theplurality of core regions is doped with a first rare-earth ion and atleast one other said section of the one said core region is doped with asecond rare-earth ion, the first rare-earth ion being different from thesecond rare earth ion.
 7. The optical fiber of claim 5, wherein at leastone said section of the plurality of sections of one said core region ofthe plurality of core regions is doped with a first doping concentrationand at least one other said section of the one said core region is dopedwith a second doping concentration, the first doping concentration beingdifferent from the second doping concentration.
 8. The optical fiber ofclaim 5, wherein at least one said section of the plurality of sectionsof one said core region of the plurality of core regions is of a firstglass host and a at least one other section of the one said core sectionis of a second glass host, the first glass host being different from thesecond glass host.
 9. A fiber laser, comprising: a laser optical fibercomprising a first core region embedded within a cladding, the firstcore region having a first plurality of sections, wherein each of thefirst plurality of sections is doped with a rare-earth ion.
 10. Thefiber laser of claim 9, wherein at least one said section of the firstplurality of sections is doped with a first rare-earth ion and at leastone other said section is doped with a second rare-earth ion, the firstrare-earth ion being different from the second rare earth ion.
 11. Thefiber laser of claim 9, wherein at least one said section of the firstplurality of sections is doped with a first doping concentration and atleast one other said section is doped with a second dopingconcentration, the first doping concentration being different from thesecond doping concentration.
 12. The fiber laser of claim 9, wherein atleast one said section of the first plurality of sections is of a firstglass host and a at least one other section is of a second glass host,the first glass host being different from the second glass host.
 13. Thefiber laser of claim 9, wherein the laser optical fiber furthercomprises a second core region, the second core region having a secondplurality of sections, wherein each of the second plurality of sectionsis doped with a rare-earth ion.
 14. An ultra short pulse fiber laser,comprising: a laser optical fiber comprising a first core regionembedded within a cladding, the first core region having a firstplurality of sections, wherein the laser optical fiber is doped with aplurality of rare-earth ions.
 15. The ultra short pulse fiber laser ofclaim 14, wherein at least one said section of the first plurality ofsections is doped with a first rare-earth ion and at least one othersaid section is doped with a second rare-earth ion, the first rare-earthion being different from the second rare earth ion.
 16. The ultra shortpulse fiber laser of claim 14, wherein at least one said section of thefirst plurality of sections is doped with a first doping concentrationand at least one other said section is doped with a second dopingconcentration, the first doping concentration being different from thesecond doping concentration.
 17. The ultra short pulse fiber laser ofclaim 14, wherein at least one said section of the first plurality ofsections is of a first glass host and a at least one other section is ofa second glass host, the first glass host being different from thesecond glass host.
 18. The ultra short pulse fiber laser of claim 14,wherein the laser optical fiber further comprises a second core region,the second core region having a second plurality of sections.
 19. Abroad band amplified spontaneous emission (“ASE”) source, comprising: anoptical fiber comprising a first core region embedded within a cladding,the first core region having a first plurality of sections wherein eachof the first plurality of sections is doped with a rare-earth ion. 20.The broad band ASE source of claim 19, wherein at least one said sectionof the first plurality of sections is doped with a first rare-earth ionand at least one other said section is doped with a second rare-earthion, the first rare-earth ion being different from the second rare earthion.
 21. The broad band ASE source of claim 19, wherein at least onesaid section of the first plurality of sections is doped with a firstdoping concentration and at least one other said section is doped with asecond doping concentration, the first doping concentration beingdifferent from the second doping concentration.
 22. The broad band ASEsource of claim 19, wherein at least one said section of the firstplurality of sections is of a first glass host and a at least one othersection is of a second glass host, the first glass host being differentfrom the second glass host.
 23. The broad band ASE source of claim 19,wherein the optical fiber further comprises a second core region, thesecond core region having a second plurality of sections.